A boat having a propulsion device of this type is disclosed in DE 2346051. In this device a drive link is connected to a forwardly extended lever that is subjected to manual pumping movements by the user for oscillating the propulsion fin. The lever is also pivotably connected to a rudder, which is pivotably connected to brackets extending from the stem of a small boat. A problem with the propulsion device of DE 2346051 is that it is difficult to keep the course of the watercraft.